Currently, Orthogonal Frequency Division Multiplexing (OFDM) is a widely used transmission scheme that has been adopted for digital terrestrial broadcasting and a variety of other digital communications, such as IEEE 802.11a. In the OFDM method, a plurality of narrow band digital modulated signals are frequency multiplexed using a plurality of orthogonal subcarriers. OFDM is therefore an excellent transmission scheme for efficiently using frequencies.
Furthermore, in the OFDM method, one symbol duration is composed of a useful symbol duration and a guard interval duration. To provide periodicity within a symbol, a signal for a portion of the useful symbol duration is copied and inserted into the guard interval duration. This allows for elimination of the effect of interference between symbols produced by multipath interference. OFDM is therefore also highly resistant to multipath interference.
In recent years, analog television broadcasting has ceased in many countries around the world, and efforts towards frequency reallocation are gaining momentum. Particularly, in Europe, in addition to Standard Definition (SD) broadcasting for Digital Video Broadcasting-Terrestrial (DVB-T), a demand for High Definition (HD) service is rising. Given these circumstances, progress has been made in the standardization of DVB-T2, the second generation of European digital terrestrial broadcasting. In the DVB-T2 system, as shown in FIG. 27, DVB-T2 frames are used. A DVB-T2 frame includes a P1 symbol, P2 symbols, and data symbols.
First, explanation is provided of a P1 symbol in a DVB-T2 frame.
A P1 symbol is set to have a Fast Fourier Transform (FFT) size of 1 k (1024). Further, as illustrated in FIG. 28, a guard interval duration is provided on both sides preceding and following the useful interval duration in a P1 symbol. Note that FIG. 28 illustrates a P1 symbol in the time domain. The guard interval durations in a P1 symbol differ from guard interval durations provided in conventional schemes such as Integrated Services Digital Broadcasting-Terrestrial (ISDB-T) and DVB-T. That is, in a P1 symbol, a signal for 59 μs from the earlier half of the useful symbol duration in the time domain, is copied and inserted into the guard interval duration that precedes the useful symbol duration (hereinafter referred to as a “preceding guard interval duration”). A signal for 53 μs from the latter half of the useful symbol duration, in the time domain, is copied and inserted into the guard interval duration that follows the useful symbol duration (hereinafter referred to as a “succeeding guard interval duration”). In addition, when copying and inserting such signals into the respective guard interval durations (the preceding guard interval duration or the succeeding guard interval duration), the original signals are frequency shifted by a predetermined frequency fSH before being inserted. Here, the predetermined frequency fSH corresponds to one subcarrier spacing in a P1 symbol. That is, the signal for the preceding guard interval duration and the signal for the succeeding guard interval duration are higher in frequency than the signal for the useful symbol duration by one subcarrier of a P1 symbol. Note that as illustrated in FIG. 28, the entire useful symbol duration is copied and used in the guard interval durations in a P1 symbol.
In addition, a P1 symbol is composed of active carriers and null carriers (unused carriers) as illustrated in FIG. 29. Note that FIG. 29 illustrates a P1 symbol in the frequency domain.
A P1 symbol includes information (hereinafter referred to as “P1 transmission information”) such as: information indicating whether the transmission format of the P2 symbols and the data symbols is Multiple-Input-Single-Output (MISO) or Single-Input-Single-Output (SISO) (hereinafter referred to as “MISO/SISO information”); information indicating the FFT size of the P2 symbols and the data symbols (hereinafter referred to as “FFT size information”); and information indicating whether Future Extension Frames (FEFs) are included (hereinafter referred to as “FEF inclusion information”). An FEF is a part reserved for future transmission services differing from DVB-T2 and is inserted in between two DVB-T2 frames. Further, a P1 symbol is located at the head of an FEF as well.
In the following, explanation is provided concerning the generation of a P1 symbol.
FIG. 30 illustrates a structure of a P1 symbol generation unit 1000 that generates a P1 symbol. The P1 symbol generation unit 1000 includes: a sequence transformation unit 1001; a differential modulation unit 1002; a scrambling unit 1003; a CDS table generation unit 1004; a padding unit 1005; an IFFT unit 1006; and a GI adding unit 1007.
As explained above, P1 transmission information is transmitted in a P1 symbol. The P1 transmission information is represented as a three-bit S1 signal and a four-bit S2 signal. The sequence transformation unit 1001 receives a three-bit S1 signal and a four-bit S2 signal as input. The sequence transformation unit 1001 stores a transform table such as the one illustrated in FIG. 31. By referring to the transform table, the sequence transformation unit 1001 transforms the three-bit S1 signal into a 64-bit sequence CSSs1, represented by Math 1 below, and the four-bit S2 signal into a 256-bit sequence CSSS2, represented by Math 2 below. Values in the “Value” column in FIG. 31 indicate values input to the sequence transformation unit 1001 and the sequences in the “Sequence (hexadecimal) CSSS1 and CSSS2” column in FIG. 31 indicate sequences after transformation (the sequences output from the sequence transformation unit 1001). Note that in FIG. 31, the transformed sequences CSSS1 and CSSS2 are represented in hexadecimal for the sake of convenience.CSSS1=(CSSS1,0, . . . ,CSSS1,63)  Math 1CSSS2=(CSSS2,0, . . . ,CSSS2,255)  Math 2
The sequence transformation unit 1001 further forms a 384-bit signal sequence MSS_SEQ indicated in Math 3 below by using the sequence CSSS1 represented by Math 1 and the sequence CSSS2 represented by Math 2 and outputs the signal sequence MSS_SEQ to the differential modulation unit 1002. Note that the signal sequence MSS_SEQ includes two identical S1 signals.
                                                        MSS_SEQ              =                            ⁢                              (                                                      MSS_SEQ                    0                                    ,                  …                  ⁢                                                                          ,                                      MSS_SEQ                    383                                                  )                                                                                        =                            ⁢                              (                                                      CSS                                          S                      ⁢                                                                                          ⁢                      1                                                        ,                                      CSS                                          S                      ⁢                                                                                          ⁢                      2                                                        ,                                      CSS                                          S                      ⁢                                                                                          ⁢                      1                                                                      )                                                                                        =                            ⁢                              (                                                      CSS                                                                  S                        ⁢                                                                                                  ⁢                        1                                            ,                      0                                                        ,                  …                  ⁢                                                                          ,                                      CSS                                                                  S                        ⁢                                                                                                  ⁢                        1                                            ,                      63                                                        ,                                      CSS                                                                  S                        ⁢                                                                                                  ⁢                        2                                            ,                      0                                                        ,                  …                  ⁢                                                                          ,                                                                                                                      ⁢                                                CSS                                                            S                      ⁢                                                                                          ⁢                      2                                        ,                    255                                                  ,                                  CSS                                                            S                      ⁢                                                                                          ⁢                      1                                        ,                    0                                                  ,                …                ⁢                                                                  ,                                  CSS                                                            S                      ⁢                                                                                          ⁢                      1                                        ,                    63                                                              )                                                          Math        ⁢                                  ⁢        3            
The differential modulation unit 1002 receives the signal sequence MSS_SEQ from the sequence transformation unit 1001 as input and performs differential modulation indicated in Math 4 below on the signal sequence MSS_SEQ. The differential modulation unit 1002 outputs a differentially modulated signal sequence MSS_DIFF to the scrambling unit 1003. The differential modulation performed by the differential modulation unit 1002 is Differential Binary Phase Shift Keying (DBPSK).MSS_DIFF=DBPSK(MSS_SEQ)  Math 4
In specific, the differential modulation unit 1002 treats a reference signal MSS_DIFF−1 as “1”, as indicated in Math 5 below, and performs differential modulation in accordance with Math 6 below on the signals MSS_SEQi (I=0, 1, . . . , 83) composing the signal sequence MSS_SEQ input from the sequence transformation unit 1001. The differential modulation unit 1002 outputs the differentially modulated signal MSS_DIFFi to the scrambling unit 1003.MSS—DIFF−1=1  Math 5
                              MSS_DIFF          i                =                  {                                                                      MSS_DIFF                                      i                    -                    1                                                                                                                    :                                          MSS_SEQ                      i                                                        =                  0                                                                                                      -                                      MSS_DIFF                                          i                      -                      1                                                                                                                                        :                                          MSS_SEQ                      i                                                        =                  1                                                                                        Math        ⁢                                  ⁢        6            
The scrambling unit 1003 performs scrambling indicated in Math 7 below on the differentially modulated signal sequence MSS_DIFF input from the differential modulation unit 1002 and outputs a scrambled signal sequence MSS_SCR to the padding unit 1005.MSS—SCR=SCRAMBLING(MSS_DIFF)  Math 7
In specific, the scrambling unit 1003 performs scrambling indicated in Math 8 below on the differentially modulated signal MSS_DIFFi by using a signal PRBSi (I=0, 1, . . . , 383) based on a Pseudo Random Binary Sequence (PRBS). The scrambling unit 1003 outputs a scrambled signal MSS_SCRi to the padding unit 1005.
                              MSS_SCR          i                =                              MSS_DIFF            i                    ×          2          ⁢                      (                                          1                2                            -                              PRBS                i                                      )                                              Math        ⁢                                  ⁢        8            
The CDS table generation unit 1004 generates a Carrier Distribution Sequence (CDS) table illustrated in FIG. 32, which indicates a position k(i) (i=0, 1, . . . , 383) of each active carrier in a P1 symbol. Note that as illustrated in FIG. 32, two identical S1 signals are transmitted in two positions in one P1 symbol, one in a high frequency range and another in a low frequency range of the P1 symbol. On the other hand, an S2 signal is transmitted in a central frequency range of the P1 symbol.
The padding unit 1005 treats a subcarrier at a subcarrier position k(i), indicated in the CDS table (refer to FIG. 32) generated by the CDS table generation unit 1004, as an active carrier. The padding unit 1005 maps the scrambled signal MSS_SCRi to a subcarrier at a subcarrier position k(i) and outputs the results of the mapping to the IFFT unit 1006. Further, the padding unit 1005 outputs subcarriers at subcarrier positions not listed in the CDS table illustrated in FIG. 32 to the IFFT unit 1006 as null carriers.
The IFFT unit 1006 performs an Inverse Fast Fourier Transform (IFFT) of a size 1 k on the signals output by the padding unit 1005. The IFFT unit 1006 then outputs the results of the IFFT (a time domain signal for the useful symbol duration illustrated in FIG. 28) to the GI adding unit 1007.
The GI adding unit 1007 uses the signal for the useful symbol duration input from the IFFT unit 1006 to frequency shift an earlier portion of the signal for the useful symbol duration by a frequency fSH and insert the result in the preceding guard interval duration, and also to frequency shift a later portion of the signal for the useful symbol duration by a frequency fSH and insert the result in the succeeding guard interval duration (refer to FIG. 28). A P1 symbol is thus generated.
Subsequently, explanation is provided of P2 symbols and data symbols.
P2 symbols and data symbols share a common FFT size and a common guard interval fraction (ratio of a time domain length of a guard interval duration to a time domain length of a useful symbol duration). Note that the guard interval duration in P2 symbols and in data symbols is provided preceding the useful symbol duration, as in DVB-T and ISDB-T. Further, a signal for a later portion of the useful symbol duration in the time domain is copied and inserted into the guard interval duration preceding the useful symbol duration.
FIG. 33 illustrates combinations of FFT size and guard interval fraction used in DVB-T2, along with pilot patterns that can be set for such combinations. There are eight pilot patterns, namely pilot patterns PP1 through PP8. In FIG. 33, “N/A” indicates a combination of FFT size and guard interval fraction that is not supported under DVB-T2.
A P2 symbol includes pilots (hereinafter referred to as “P2 pilots”) inserted at equal intervals. In specific, one P2 pilot exists every six subcarriers for FFT size of 32 k and in SISO mode. Otherwise, one P2 pilot exists every three subcarriers.
A P2 symbol includes any transmission parameter information required for reception (hereinafter referred to as “P2 transmission information”). The P2 transmission information includes information such as: information indicating the pilot pattern of the data symbols (hereinafter referred to as “pilot pattern information”); information indicating whether the carrier extension mode is Extended mode or Normal mode (hereinafter referred to as “transmission mode information”); information indicating the number of symbols included per frame; information indicating the modulation method; and information indicating a coding ratio of the Forward Error Correction (FEC) code. Note that the number of P2 symbols included per DVB-T2 frame is set according to the FFT size of the P2 symbols as illustrated in FIG. 34.
Non-Patent Literature 1 provides one technology for demodulating P1 symbols in the above-described DVB-T2 transmission format.
FIG. 35 illustrates a structure of a P1 demodulation unit 2000 that demodulates P1 symbols. The P1 demodulation unit 2000 includes: a P1 position detection unit 2001; a P1 narrow band fc error detection/correction unit 2002; an FFT unit 2003; a CDS table generation unit 2004; a P1 wide band fc error detection/correction unit 2005; and a P1 decoding unit 2006.
The P1 position detection unit 2001 uses input signals and calculate correlations (guard correlations) between a signal for the guard interval duration (the preceding guard interval duration and the succeeding guard interval duration) and a signal for a predetermined portion of the useful symbol duration of the P1 symbol. The P1 position detection unit 2001 calculates the integral of the calculated correlations over the time domain length of the guard interval duration (the preceding guard interval duration and the succeeding guard interval duration) and detects the position of the P1 symbol in the input signals by detecting the peak value of the integral.
The calculation of the correlations takes into consideration a frequency shift of a frequency fSH that is added at the transmitting end. In addition, the “predetermined portion” of the useful symbol duration is the earlier portion of the useful symbol duration for the preceding guard interval duration and is the later portion of the useful symbol duration for the succeeding guard interval duration (refer to FIG. 28). The same applies for the calculation of the correlations performed by the P1 narrow band fc error detection/correction unit 2002 described in the following.
The P1 narrow band fc error detection/correction unit 2002 calculates the correlations (guard correlations) between the signal for the guard interval duration (the preceding guard interval duration and the succeeding guard interval duration) and the signal for a predetermined portion of the useful symbol duration of the P1 symbol. According to the correlations, the P1 narrow band fc error detection/correction unit 2002 detects a frequency error amount (a narrow band carrier frequency error amount) that is equal to or smaller than one subcarrier spacing of the P1 symbol. According to the narrow band carrier frequency error amount so detected, the P1 narrow band fc error detection/correction unit 2002 corrects a narrow band carrier frequency shift of the P1 symbol and outputs the P1 symbol whose narrow band carrier frequency shift has been corrected to the FFT unit 2003.
The FFT unit 2003 performs an FFT of size 1 k on the time domain signal for the useful symbol duration of the P1 symbol and outputs the results of the FFT (a frequency domain signal of the useful symbol duration of the P1 symbol) to the P1 wide band fc error detection/correction unit 2005.
The CDS table generation unit 2004 generates a sequence indicating the positions of active carriers in the P1 symbol (hereinafter referred to as an “active carrier arrangement sequence”) and outputs the active carrier arrangement sequence so generated to the P1 wide band fc error detection/correction unit 2005. The active carrier arrangement sequence is a sequence with a “1” at positions corresponding to active carriers, as indicated in FIG. 32, and a “0” at other positions corresponding to null carriers.
The P1 wide band fc error detection/correction unit 2005 uses the active carrier arrangement sequence input from the CDS table generation unit 2004 and detects a frequency error amount (a wide band carrier frequency error amount) in units of a subcarrier spacing of the P1 symbol of the signal output by the FFT unit 2003. According to the wide band carrier frequency error amount so detected, the P1 wide band fc error detection/correction unit 2005 corrects a wide band carrier frequency shift of the P1 symbol. The P1 wide band fc error detection/correction unit 2005 extracts only active carriers in the P1 symbol whose wide band carrier frequency shift has been corrected and outputs the active carriers so extracted to the P1 decoding unit 2006.
In the following, explanation is provided concerning the detection of the wide band carrier frequency error amount for the P1 symbol. As described above, subcarriers composing a P1 symbol are either active carriers or null carriers. By taking advantage of this structure, the P1 wide band fc error detection/correction unit 2005 calculates the power of each subcarrier, and further, calculates the correlations (arrangement correlations) between the results of the calculation and a known active carrier arrangement sequence (input from the CDS table generation unit 2004) while shifting the results of the calculation one subcarrier at a time.
Since signals on which DBPSK has been performed are mapped to the active carriers, the arrangement correlation value for a shift amount corresponding to a wide band carrier frequency error amount is the sum of the power values of all active carriers. This arrangement correlation value is a greater value than the arrangement correlation values for other shift amounts, which include power values of null carriers. As such, the shift amount yielding the greatest arrangement correlation value is determined as the wide band carrier frequency error amount. It is thus possible to detect the wide band carrier frequency error amount. Note that the shift amount when there is no wide band carrier frequency error amount in the input signal is treated as a reference (shift amount “0”) here and in the following description.
The P1 decoding unit 2006 illustrated in FIG. 35 decodes the P1 symbol according to the active carriers in the P1 symbol input from the P1 wide band fc error detection/correction unit 2005 and extracts the P1 transmission information.
Explanation is provided of the P1 decoding unit 2006 with reference to FIG. 36. FIG. 36 illustrates a structure of the P1 decoding unit 2006 illustrated in FIG. 35. The P1 decoding unit 2006 includes: a descrambling unit 2101; a differential demodulation unit 2102; and a pattern matching unit 2103. Note that here, the P1 decoding unit 2006 decodes a P1 symbol by using only the S1 signal included in the low frequency range of the P1 symbol.
The descrambling unit 2101 receives a signal sequence Act of the active carriers of the P1 symbol from the P1 wide band fc error detection/correction unit 2005 illustrated in FIG. 35 as input. The descrambling unit 2101 performs the descrambling indicated in Math 9 below on the active carrier signal sequence Act and outputs a descrambled signal sequence DESCR to the differential demodulation unit 2102.DESCR=DESCRAMBLING(Act)  Math 9
In specific, the descrambling unit 2101 performs descrambling as indicated in Math 10 below on the active carrier signal Act by using a signal PRBSi (I=0, 1, . . . , 319), based on a PRBS used for multiplication at the transmitting end, and outputs a descrambled signal DESCRi to the differential demodulation unit 2102.
                              DESCR          i                =                              Act            i                    ×          2          ⁢                      (                                          1                2                            -                              PRBS                i                                      )                                              Math        ⁢                                  ⁢        10            
The differential demodulation unit 2102 receives the descrambled signal DESCRi (i=0, 1, . . . , 319) from the descrambling unit 2101 as input. The differential demodulation unit 2102 performs differential detection by complex multiplication of the signal DESCRi (i=0, 1, . . . , 319) and a signal DESCR*i-1, which is the complex conjugate of a signal DESCRi-1 obtained by shifting the signal DESCRi by one active carrier. Note that the suffix “*” in superscript represents a complex conjugate (the same applying in the following as well). Further, the differential demodulation unit 2102 demodulates (hard decision) the signal DESCRi·DESCR*i-1 according to the polarity of the real axis of the result of the differential detection and outputs the demodulated signal DEMODi to the pattern matching unit 2103. The processing by the differential demodulation unit 2102 is represented by Math 11 below. The differential demodulation by the differential demodulation unit 2102 is demodulation corresponding to DBPSK.
                              DEMOD          i                =                  {                                                                      0                  :                                                                          ⁢                                                            real                      ⁢                                                                                          ⁢                                              (                                                                              DESCR                            i                                                    ·                                                      DESCR                                                          i                              -                              1                                                        *                                                                          )                                                              ≥                    0                                                                                                                        1                  :                                                                          ⁢                                                            real                      ⁢                                                                                          ⁢                                              (                                                                              DESCR                            i                                                    ·                                                      DESCR                                                          i                              -                              1                                                        *                                                                          )                                                              <                    0                                                                                                          Math        ⁢                                  ⁢        11            
Since i=0 is a reference, the differential demodulation unit 2102 performs demodulation (hard decision) according to the polarity of the real axis of the signal DESCR0 and outputs the demodulated signal DEMOD0 to the pattern matching unit 2103.
The pattern matching unit 2103 divides the signals DEMOD0, DEMOD1, . . . , DEMOD319 differentially demodulated by the differential demodulation unit 2102 into a signal sequence DEMOD_CSSS1 (corresponding to the S1 signal) and a signal sequence DEMOD_CSSS2 (corresponding to the S2 signal) as indicated in Math 12 and Math 13 below.
                    ⁢          Math      ⁢                          ⁢      12                                                DEMOD_CSS                          S              ⁢                                                          ⁢              1                                =                    ⁢                      (                                          DEMOD                0                            ,              …              ⁢                                                          ,                              DEMOD                63                                      )                                                        =                    ⁢                      (                                          DEMOD_CSS                                                      S                    ⁢                                                                                  ⁢                    1                                    ,                  0                                            ,              …              ⁢                                                          ,                              DEMOD_CSS                                                      S                    ⁢                                                                                  ⁢                    1                                    ,                  63                                                      )                                                  ⁢          Math      ⁢                          ⁢      13                                                DEMOD_CSS                          S              ⁢                                                          ⁢              2                                =                    ⁢                      (                                          DEMOD                64                            ,              …              ⁢                                                          ,                              DEMOD                319                                      )                                                        =                    ⁢                      (                                          DEMOD_CSS                                                      S                    ⁢                                                                                  ⁢                    2                                    ,                  0                                            ,              …              ⁢                                                          ,                              DEMOD_CSS                                                      S                    ⁢                                                                                  ⁢                    2                                    ,                  255                                                      )                              
Further, the pattern matching unit 193 performs the following processing to calculate which of the sequences CSSS1,k (k=0, 1, . . . , 7) indicated in FIG. 31 has the highest degree of certainty and to calculate which of the signal sequences CSSS2,k (k=0, 1, . . . , 15) indicated in FIG. 31 has the highest degree of certainty. In this context, the index k is used to differentiate the 8 sequences CSSS1 indicated in FIG. 31 and to differentiate the 16 sequences CSSS2 indicated in FIG. 31 (the same applying in the following as well).
The pattern matching unit 2103 calculates correlations CORRS1,k between the sequences CSSS1,k indicated in FIG. 31 and the sequence DEMOD_CSSS1, as indicated in Math 14 below. The pattern matching unit 2103 also calculates correlations CORRS2,k between the sequences CSSS2,k indicated in FIG. 31 and the sequence DEMOD_CSSS2, as indicated in Math 15 below.
                              CORR                                    S              ⁢                                                          ⁢              1                        ,            k                          =                              ∑                          i              =              0                        63                    ⁢                                    DEMOD_CSS                                                S                  ⁢                                                                          ⁢                  1                                ,                i                                      ⊕                          CSS                                                S                  ⁢                                                                          ⁢                  1                                ,                k                ,                i                                      ⁢                                                  ⊕                                                  ⁢                          indicates              ⁢                                                          ⁢              exclusive              ⁢                                                          ⁢              or                                                          Math        ⁢                                  ⁢        14                                          CORR                                    S              ⁢                                                          ⁢              2                        ,            k                          =                              ∑                          i              =              0                        255                    ⁢                                    DEMOD_CSS                                                S                  ⁢                                                                          ⁢                  2                                ,                i                                      ⊕                          CSS                                                S                  ⁢                                                                          ⁢                  2                                ,                k                ,                i                                      ⁢                                                  ⊕                                                  ⁢                          indicates              ⁢                                                          ⁢              exclusive              ⁢                                                          ⁢              or                                                          Math        ⁢                                  ⁢        15            
The pattern matching unit 2103 estimates that the three-bit S1 signal (refer to FIG. 31) corresponding to the signal sequence CSSS1,k having the greatest correlation value among the 8 correlation values calculated by using Math 14 above is the transmitted S1 signal. The pattern matching unit 2103 also estimates that the four-bit S2 signal (refer to FIG. 31) corresponding to the signal sequence CSSS2,k having the greatest correlation value among the 16 correlation values calculated by using Math 15 above is the transmitted S2 signal. The pattern matching unit 2103 obtains the P1 transmission information by using the S1 signal and the S2 signal so estimated.